Method for manufacturing products made of magnesium-lithium-aluminum alloy

ABSTRACT

The invention relates to a method for manufacturing a wrought product. The invention further provides, a wrought product able to be obtained according to the method of the invention as well as the use of said wrought product to produce aircraft structural elements.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a wrought product made of aluminum-magnesium-lithium alloy, more particularly a method for manufacturing such a product having an improved compromise of properties, in particular an improved compromise between the tensile yield strength and the toughness of said products. The invention has also for object a product able to be obtained by said method for manufacturing and its use, said product being intended in particular for aeronautical and aerospace construction.

PRIOR ART

Wrought products made of an aluminum alloy are developed to produce high-resistance parts intended in particular for the aeronautical industry and the aerospace industry.

Aluminum alloys containing lithium are very interesting in this respect, as lithium can reduce the density of the aluminum by 3% and increase the elastic modulus by 6% for each percentage in weight of lithium added. In particular, aluminum alloys containing simultaneously magnesium and lithium make it possible to reach particularly low densities and therefore have been studied extensively.

Patent GB 1.172.736 discloses an alloy containing, in percentage by weight, 4 to 7% of Mg, 1.5-2.6% of Li, 0.2-10 of Mn and/or 0.05-0.3% of Zr, the remainder is aluminum. This alloy is useful for the elaboration of products that have a high mechanical resistance, good corrosion resistance, low density and a high elastic modulus. Said products are obtained by a method comprising an optional quenching then an artificial aging. By way of example, the products coming from the method according to GB 1.172.736 have a ultimate tensile strength ranging from about 440 MPa to about 490 MPa, a tensile yield strength ranging from about 270 MPa to about 340 MPa and an elongation to fracture of about 5-8%.

International application WO 92/03583 describes an alloy useful for aeronautical structures that have a low density and general formula Mg_(a)Li_(b)Zn_(c)Ag_(d)Al_(bal), wherein a is between 0.5 and 10%, b is between 0.5 and 3%, c is between 0.1 and 5%, d is between 0.1 and 2% and bal indicates that the remainder is aluminum. This document also discloses a method for obtaining said alloy comprising the steps of: a) casting an ingot which has the composition described hereinabove, b) removing the residual stresses from said ingot by thermal treatment, c) homogenizing by heating and maintaining at temperature then cooling the ingot, d) hot rolling said ingot to its final thickness, e) solution heat treated then quenching the product rolled as such, f) stretching the product and g) carrying out an artificial aging of said product by heating and maintaining at temperature.

U.S. Pat. No. 5,431,876 discloses a ternary group of alloys of aluminum lithium and magnesium or copper, including at least one additive such as zirconium, chromium and/or manganese. The alloy is prepared according to methods known to those skilled in the art comprising, by way of example, an extrusion, a solution heat treatment, a quenching, a stretching of the product from 2 to 7% then an artificial aging.

U.S. Pat. No. 6,551,424 describes a method for manufacturing rolled products made of aluminum-magnesium-lithium alloy having the composition (in % by weight) Mg: 3.0-6.0; Li: 0.4-3.0; Zn up to 2.0; Mn up to 1.0; Ag up to 0.5; Fe up to 0.3; Si up to 0.3; Cu up to 0.3; 0.02-0.5 of an element selected from the group comprising Sc, Hf, Ti, V, Nd, Zr, Cr, Y, Be, said method including a cold rolling lengthwise and widthwise.

U.S. Pat. No. 6,461,566 describes an alloy having the composition (in % by weight) Li: 1.5-1.9; Mg: 4.1-6.0; Zn 0.1-1.5; Zr 0.05-0.3; Mn 0.01-0.8; H 0.9×10⁻⁵-4.5×10⁻⁵ and at least one element selected from the group of Be 0.001-0.2; Y 0.001-0.5 and Sc 0.01-0.3.

Patent application WO 2012/16072 describes a wrought product made of an aluminum alloy having the composition in % by weight, Mg: 4.0-5.0; Li: 1.0-1.6; Zr: 0.05-0.15; Ti: 0.01-0.15; Fe: 0.02-0.2; Si: 0.02-0.2; Mn: ≦0.5; Cr ≦0.5; Ag: ≦0.5; Cu ≦0.5; Zn ≦0.5; Sc <0.01; other elements <0.05; the remainder is aluminum. Said product is in particular obtained according to a method for manufacturing comprising in particular successively the casting of the alloy in unprocessed form, hot working optionally cold working of it, solution heat treatment then the quenching the wrought product, optionally the cold working of the solution heat treated and quenched product as such and finally the artificial aging of the wrought product at a temperature less than 150° C. The temper obtained for rolled products is advantageously a T6 or T6X or T8 or T8X temper and for extruded products advantageously a T5 or T5X temper in the case of press quenching or a T6 or T6X or T8 or T8X temper.

The wrought products made of aluminum-magnesium-lithium alloy have a low density and are therefore particularly interesting in the extremely demanding field of aeronautics. In order for new products to be selected in such a field, their performance has to be significantly improved in relation to that of existing products, in particular their performance in terms of a compromise between the static mechanical resistance properties (in particular tensile and compression yield strength, ultimate tensile strength) and damage tolerance properties (toughness, resistance to fatigue crack propagation), with these properties being mutually exclusive in general.

These alloys must also have sufficient corrosion resistance, in order to be formed according to the usual methods and have low residual stresses so as to be able to be machined without substantial distortion during said machining.

There is therefore a need for wrought products made of aluminum-magnesium-lithium alloy that have a low density as well as improved properties in relation to those of known products, in particular in terms of a compromise between the static mechanical resistance properties and the damage tolerance properties. With regards to the damage tolerance properties, wrought products must in particular have a high toughness as well as a low propensity for delamination. Such products must in addition be able to be obtained according to a method of manufacturing that is reliable, economical and easy to adapt to a conventional manufacturing line.

OBJECT OF THE INVENTION

A first object of the invention is a method for manufacturing a wrought product wherein:

(a) an unprocessed form of aluminum alloy is cast, which has the composition, in % by weight: Mg: 4.0-5.0; Li: 1.0-1.8; Zr: 0.05-0.15; Mn: ≦0.6; Ag: ≦0.5; Fe: ≦0.1; Ti: <0.15; Si: ≦0.05; other elements ≦0.05 each and ≦0.15 in association; the remainder is aluminum;

(b) optionally, said unprocessed form is homogenized;

(c) said unprocessed form is hot worked to obtain a hot-worked product;

(d) optionally, said hot-worked product is solution heat treated at a temperature from 360° C. to 460° C., preferably from 380 to 420° C., for 15 minutes to 8 hours;

(e) said hot-worked product is quenched;

(f) optionally, said hot-worked and quenched product is straightened or flattened;

(g) said hot-worked and quenched product is artificially aged;

(h) the hot-worked product artificially aged as such is cold worked in a controlled manner in order to obtain a permanent cold working set from 1 to 10%, preferably from 2 to 6%, most preferably from 3 to 5% and, most preferably from 4 to 5%.

The invention further has for objects, a wrought product able to be obtained according to the method of the invention as well as the use of said wrought product in order to carry out an aircraft structural element.

DESCRIPTION OF THE FIGURES

FIG. 1: Profile for circumferential frame of example 1

FIG. 2: Tensile yield strength, Rp0.2, according to the toughness, K_(Q)* for a flat bar 10 mm thick (* all of the values of K_(Q) are invalid due to the P_(max)/P_(Q)≦1.10 criterion of standard ASTM E399)

FIG. 3: Tensile yield strength, Rp0.2, according to the stress intensity factor corresponding to the maximum force, K_(max) (evaluated according to standard ASTM E399) for a flat bar 10 mm thick

DESCRIPTION OF THE INVENTION

Unless mentioned otherwise, all of the indications concerning the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. By way of example, the expression 1.4 Cu means that the copper content expressed in % by weight is multiplied by 1.4. The designation of the alloys is carried out in accordance with the regulations of The Aluminum Association, known to those skilled in the art. The density depends on the composition and is determined by calculation rather than by a method of measuring weight. The values are calculated in accordance with the procedure of The Aluminum Association, which is described on pages 2-12 and 2-13 of “Aluminum Standards and Data”. The definitions of the tempers are indicated in European standard EN 515.

The tensile static mechanical characteristics, in other terms the ultimate tensile strength R_(m), the conventional tensile yield strength at 0.2% R_(p0.2), and the elongation to fracture A %, are determined by a tensile test according to standard NF EN ISO 6892-1, the sampling and the direction of the test are defined by standard EN 485-1.

The toughness is determined by toughness test K1c according to standard ASTM E399. A curve providing the effective stress intensity factor according to the effective crack growth is determined according to standard ASTM E399. The tests were carried out with a specimen CT8 (B=8 mm, W=mm). In the case of values of K_(Q) invalid according to standard ASTM E399, in particular in relation to criterion P_(max)/P_(Q)≦1.10, the results were also presented in K_(max) (stress intensity factor corresponding to the maximum force P_(max)).

The increase in the stresses on the product during the toughness test K1c according to standard ASTM E399 can reveal the propensity of the product for delamination. Here, “delamination” (also “crack delamination” and/or “crack divider”) means a cracking in the planes orthogonal to the front of the main crack. The orientation of these plans corresponds to that of the seals of the non-recrystallized grains after deformation via working. Low delamination is the sign of lesser fragility of the planes concerned and minimized the risks of a deviation of a crack towards the longitudinal direction during fatigue propagation or under monotonous stress.

Unless mentioned otherwise, the definitions of standard EN 12258 apply.

Moreover, here “structure element” or “structural element” of a mechanical construction a mechanical part for which the static and/or dynamic mechanical properties are particularly substantial for the performance of the structure and for which a calculation structure is usually prescribed or carried out. This is typically elements of which the failure is able to jeopardize the safety of said construction, of its users or of other persons. For an aircraft, these structural elements include in particular the elements that comprise the fuselage (such as the fuselage skin, fuselage stringers, bulkheads, circumferential frames, wings (such as the upper or lower wing skin, stringers or stiffeners, ribs, spars, floor beams and seat tracks) and tail plane comprised in particular of horizontal or vertical stabilizers, as well as the doors.

The method for manufacturing products according to the invention comprises the successive steps of elaborating a liquid metal bath in such a way as to obtain an Al—Mg—Li alloy having a particular composition, the casting of said alloy in unprocessed form, optionally the homogenization of said unprocessed form cast as such, the hot working of said unprocessed form in order to obtain a hot-worked product, optionally the separate solution heat treatment of the product hot worked as such, the quenching of said hot-worked product, optionally the straightening/flattening of the worked and quenched product, the artificial aging of said worked and quenched product and the cold working in a controlled manner of the aged product in order to obtain a permanent cold working set from 1 to 10%, preferably from 2 to 6%, most preferably from 3 to 5% and most preferably from 4 to 5%.

The method for manufacturing therefore consists first of all in the casting of an unprocessed form of Al—Mg—Li alloy having the composition, in % by weight: Mg: 4.0-5.0; Li: 1.0-1.8; Zr: 0.05-0.15; Mn: ≦0.6; Ag: ≦0.5; Fe: ≦0.1; Ti: <0.15; Si: ≦0.05; other elements ≦0.05 each and ≦0.15 in association; the remainder is aluminum. A liquid metal bath is therefore carried out then cast in unprocessed form, typically a rolling ingot, an extrusion billet or a forging blank.

According to an advantageous embodiment, the Al—Mg—Li alloy has a Mn content, in % by weight, from 0.2 to 0.6%, preferably from 0.35 to 0.5%, more preferably from 0.35 to 0.45% and most preferably from 0.35 to 0.40%.

The products made of an alloy such as described hereinabove and having the advantageous Mn content have in particular improved static mechanical properties as well as a low propensity for delamination.

According to an advantageous embodiment, the unprocessed form made of aluminum alloy has a silver content less than or equal to 0.25% by weight, more preferably a silver content from 0.05% to 0.1% by weight. This element contributes in particular to the static mechanical properties. In addition, according to a more advantageous embodiment, the unprocessed form made of aluminum alloy has a total Ag and Cu content less than 0.15% by weight, preferably less than or equal to 0.12%. Controlling the maximum content in these two elements in association makes it possible in particular to improve the intergranular corrosion resistance of the wrought product.

According to a particular embodiment, the unprocessed form has a zinc content, in % by weight, less than 0.04%, preferably less than or equal to 0.03%. Such a limitation of the zinc content in the particular alloy described hereinabove yielded excellent results in terms of density and corrosion resistance of the alloy.

According to another embodiment compatible with the preceding modes, the unprocessed form made of aluminum alloy has an Fe content, in % by weight, less than 0.08%, preferably less than or equal to 0.07%, most preferably less than or equal to 0.06%. These inventors think that a minimum Fe content, as well as that possibly of Si, can contribute to improving the mechanical properties and in particular the fatigue properties of the alloy. Excellent results were in particular obtained for a Fe content from 0.02 to 0.06% by weight and/or a Si content from 0.02 to 0.05% by weight.

The lithium content of the products according to the invention is between 1.0 and 1.8% by weight. According to an advantageous embodiment, the unprocessed form made of aluminum alloy has a Li content, in % by weight, less than 1.6%, preferably less than or equal to 1.5%, most preferably less than or equal to 1.4%. A minimum lithium content of 1.1% by weight and preferably from 1.2% by weight is advantageous. These inventors have observed that a limited lithium content, in the presence of certain alloying elements, makes it possible to improve the toughness very significantly, which largely compensates for the slight increase in density and the decrease in the static mechanical properties.

According to a preferred embodiment, the unprocessed form made of aluminum alloy has a Zr content, in % by weight, from 0.10 to 0.15%. The inventors indeed observed that such a Zr content makes it possible to obtain an alloy that has a fiber structure favorable for improved static mechanical properties.

According to an advantageous embodiment, the unprocessed form made of aluminum alloy has a Mg content, in % by weight, from 4.5 to 4.9%. Excellent results were obtained for alloys according to this embodiment in particular regarding the static mechanical properties.

According to an advantageous embodiment, the Cr content of the products according to the invention is less than 0.05% by weight, preferably less than 0.01% by weight. Such a limited Cr content in association with the other elements of the alloy according to the invention makes it possible in particular to limit the forming of primary phases during the casting.

The Ti content of the products according to the invention is less than 0.15% by weight, preferably between 0.01 and 0.05% by weight. The Ti content is limited in the particular alloy of this invention in particular to prevent the forming of primary phases during the casting. On the other hand, it can be advantageous to control the Ti content in order to control the granular structure and in particular the grain size during the casting of the alloy.

Certain elements can be harmful for Al—Mg—Li alloys such as described hereinabove, in particular for reasons of transformation of the alloy such as toxicity and/or fracture during the working. It is therefore preferable to limit these elements to a very low level, i.e. less than 0.05% by weight or even less. In an advantageous embodiment, the products according to the invention have a maximum content of 10 ppm of Na, preferably from 8 ppm of Na, and/or a maximum content of ppm of Ca. According to a particularly advantageous embodiment, the unprocessed form made of aluminum alloy is substantially free of Sc, Be, Y, more preferably said unprocessed form comprises less than 0.01% by weight of these elements taken in combination.

According to a particularly advantageous embodiment, the unprocessed form made of aluminum alloy has a composition, in % by weight:

Mg: 4.0-5.0, preferably 4.5-4.9;

Li: 1.1-1.6, preferably 1.2-1.5;

Zr: 0.05-0.15, preferably 0.10-0.15;

Ti: <0.15, preferably 0.01-0.05;

Fe: 0.02-0.1, preferably 0.02-0.06;

Si: 0.02-0.05;

Mn: ≦0.6, preferably 0.2-0.6, most preferably 0.35-0.5;

Cr: <0.05, preferably <0.01;

Ag: ≦0.5; preferably ≦0.25; most preferably ≦0.1;

Sc: <0.01;

other elements ≦0.05 each and ≦0.15 in association;

the remainder is aluminum. Excellent results have been obtained with an alloy having such a composition.

Following the step of casting the unprocessed form, the method for manufacturing optionally comprises a step of homogenizing the unprocessed form in such a way as to reach a temperature between 450° C. and 550° C. and, preferably, between 480° C. and 520° C. for a period of time between 5 and 60 hours. The homogenization treatment can be carried out in one or several steps. According to a preferred embodiment of the invention, the hot working is carried out directly following a simple heating without carrying out any homogenization.

The unprocessed form is then hot worked, typically by extrusion, rolling and/or forging, in order to obtain a worked product. This hot working is carried out at an inlet temperature greater than 400° C. and, advantageously, from 420° C. to 450° C. According to an advantageous embodiment, the hot working is a working via extrusion of the unprocessed form.

In the case of manufacturing plate by rolling, it may be necessary to carry out a step of cold rolling (which then constitutes a first optional step of cold working) for the products of which the thickness is less than 3 mm. It may be useful to carry out one or several intermediate thermal treatments, typically carried out at a temperature between 300 and 420° C., before or during the cold rolling.

The hot-worked and optionally cold-worked, product is optionally subjected to a separate solution heat treatment at a temperature from 360° C. to 460° C., preferably from 380° C. to 420° C., for 15 minutes to 8 hours.

The hot-worked product and, optionally, solution heat treated is then quenched. The quenching is carried out with water and/or with air. It is advantageous to carry out the quenching with air as the intergranular corrosion properties are improved. In the case of an extruded product, it is advantageous to carry out the press quenching (or quenching using the extrusion heat), preferably an air press quenching, with such a quenching making it possible in particular to improve the static mechanical properties. According to another embodiment, this can also be a water press quenching. In the case of press quenching, the product is solution heat treated using the extrusion heat.

The hot-worked and quenched product can possibly be subjected to a step of straightening or flattening according to whether it is a profile or plate. Here, “straightening/flattening” means a step of cold working without permanent set or with permanent set less than 1%.

The hot-worked, quenched and, optionally straightened/flattened product, then undergoes a step of artificial aging. Advantageously, the artificial aging is carried out by heating, in one or several steps, at a temperature less than 150° C., preferably at a temperature from 70° C. to 140° C., for 5 to 100 hours.

Finally, the hot-worked product aged as such is cold worked in a controlled manner in order to obtain a permanent cold working set from 1 to 10%, preferably from 2 to 6%, most preferably from 3 to 5% and, most preferably from 4 to 5%. According to an advantageous embodiment, the permanent cold working set is from 2 to 4%. The cold working can in particular be carried out by stretching, compression and/or rolling. According to a preferred embodiment, the cold working is carried out by stretching. Entirely unexpectedly, it was revealed that, when it is carried out after the step of artificial aging, the cold working in a controlled manner of a wrought product having the composition such as described hereinabove makes it possible to obtain an excellent compromise between the static mechanical properties and those of damage tolerance, in particular toughness. The temper obtained for the wrought products corresponds in particular a T9 temper according to standard EN515. According to an advantageous embodiment, the method for manufacturing a wrought product does not comprise any step of cold working inducing a permanent working set of at least 1% between the step of hot working or, if this step is present, of solution heat treatment and the step of artificial aging.

The combination of the chosen composition, in particular of the content in Mg, Li and Mn if the latter is present, and of the transformation parameters, in particular the order of the steps of the method of manufacturing, advantageously makes it possible to obtain wrought products having a compromise of improved properties that is entirely particular, in particular the compromise between the mechanical resistance and the damage tolerance, while still having a low density and a good corrosion performance.

The wrought products according to the invention are preferably extruded products such as profiles, rolled products such as plate or thick plate and/or forged products.

The invention also has for object wrought products able to be obtained according to the method described hereinabove, advantageously such cold-worked products with a permanent cold working set greater than 4%. Indeed, such products have entirely new and particular characteristics.

The wrought products able to be obtained by the method according to the invention, advantageously said products with a permanent cold working set greater than 4%, have, in particular at mid-thickness, for a thickness between 0.5 and 15 mm, at least a static mechanical resistance property chosen from the properties (i) to (iii) and at least one damage tolerance property chosen from the properties (iv) to (v):

(i) an ultimate tensile strength Rm (L)≧440 MPa, preferably Rm (L)≧445 MPa, more preferably Rm (L)≧450 MPa and, most preferably Rm (L)≧465 MPa;

(ii) a tensile yield strength Rp0.2 (L)≧360 MPa, preferably Rp0.2 (L)≧380 MPa, more preferably Rp0.2 (L)≧390 MPa and, most preferably, Rp0.2 (L)≧400 MPa;

(iii) a tensile yield strength Rp0.2 (TL)≧330 MPa and preferably Rp0.2 (TL)≧340 MPa and, most preferably, Rp0.2 (TL)≧370 MPa;

(iv) a toughness, measured according to standard ASTM E399 with specimens CT8 of width W=16 mm and of thickness=8 mm, KQ (L-T)≧20 MPa√m, preferably KQ (L-T)≧22 MPa√m;

(v) a stress intensity factor corresponding to the maximum force Pmax, measured according to standard ASTM E399 with specimens CT8 of width W=16 mm and of thickness=8 mm, Kmax (L-T)≧20 MPa√m, preferably Kmax (L-T)≧25 MPa√m.

According to a preferred embodiment, the wrought products able to be obtained by the method according to the invention have, for a thickness between 0.5 and 15 mm, at mid-thickness at least two static mechanical resistance properties chosen from the properties (i) to (iii) and at least one damage tolerance properties chosen from the properties (iv) to (v).

The wrought products according to the invention furthermore have a lesser propensity for delamination, with the latter being evaluated on the fracture surfaces of specimens K1c according to standard ASTME399 (specimen CT8, B=8 mm, W=16 mm).

The extruded products according to the invention have particularly advantageous characteristics. The extruded products preferably have a thickness between 0.5 mm and 15 mm, but products with a thickness greater than 15 mm, up to 50 mm or even 100 mm or more can also have advantageous properties. The thickness of the extruded products is defined according to standard EN 2066: 2001: the transversal section is divided into elementary rectangles with dimensions A and B; with A always being the greatest dimension of the elementary rectangle and B able to be considered as the thickness of the elementary rectangle. The bottom is the elementary rectangle that has the greatest dimension A.

The wrought products according to the invention are advantageously used to carry out aircraft structural elements, in particular of planes. Preferred aircraft structural elements are in particular a fuselage skin, a circumferential frame, a fuselage stiffener or stringer or a wing skin, a wing stiffener, a rib or a spar. These aspects, as well as others of the invention are explained in more detail using the following illustrative and non-limiting examples.

EXAMPLES Example 1

Several unprocessed forms made of Al—Mg—Li alloy of which the composition is provided in table 1 have been cast. Alloys A and B both have a composition suitable for the implementation of the method according to the invention. The density of alloys A and B, calculated in conformity with the procedure of The Aluminum Association described in pages 2-12 and 2-13 of “Aluminum Standards and Data”, is 2.55.

TABLE 1 Composition in % by weight and density of the Al—Mg—Li alloys used Na Ca Alloy Ag Li Si Fe Cu Ti Mn Mg Zn Zr (ppm) (ppm) Density A 0.10 1.39 0.04 0.05 0.01 0.03 0.14 4.56 0.03 0.12 8 22 2.55 B 0.11 1.39 0.03 0.06 0.01 0.03 0.41 4.57 0.03 0.11 8 15 2.55

Billets 358 mm in diameter were carried out in the unprocessed forms. They were heated to 430-440° C. then hot worked by extrusion on a press in the form of a profile for circumferential frame such as shown in FIG. 1. The products extruded as such were quenched with air (press quenching). They were then subjected to:

-   -   for products in final T6 temper: a two-step artificial aging         carried out for 30 h at 120° C. followed by 10 h at 100° C.;     -   for products in final T8 temper: a controlled stretching with         permanent working set of 3 or 5% (respectively T8-3% and T8-5%)         then a two-step artificial aging carried out for 30 h at 120° C.         followed by 10 h at 100° C.;     -   for products in final T9 temper: a two-step artificial aging         carried out for 30 h at 120° C. followed by 10 h at 100° C. then         a controlled stretching with permanent working set of 3 or 5%         (respectively T9-3% and T9-5%).

Samples were tested in order to determine their static mechanical properties (tensile yield strength R_(p0.2) in MPa, ultimate tensile strength Rm in MPa, and elongation A in %).

The results obtained are given in tables 2 (direction L) and 3 (direction TL) hereinbelow. These results are the averages of 4 measurements taken on full thickness samples sampled on 4 positions on the circumferential frame (positions referenced as a, b, c and d in FIG. 1) for the direction L and of 2 measurements taken on full thickness samples sampled on 1 single position, referenced as c in FIG. 1, for the direction TL.

TABLE 2 Mechanical properties of the products obtained (direction L) Mechanical Alloy property T6 T8 (3%) T8 (5%) T9 (3%) T9 (5%) A Rm (MPa) 427 447 449 449 460 Rp0.2 (MPa) 298 330 346 382 409 A (%) 12 11 10 9 9 B Rm (MPa) 441 453 459 460 468 Rp0.2 (MPa) 321 340 354 398 418 A (%) 11 10 9 8 7

TABLE 3 Mechanical properties of the products obtained (direction TL) Mechanical Alloy property T6 T8 (3%) T8 (5%) T9 (3%) T9 (5%) A Rm (MPa) 441 441 439 441 454 Rp0.2 (MPa) 308 308 302 335 371 A (%) 14 14 13 11 9 B Rm (MPa) 444 456 459 456 467 Rp0.2 (MPa) 298 309 333 328 347 A (%) 15 14 14 10 11

The mechanical properties, in particular the maximum stress that can be withstood by the product or ultimate tensile strength, Rm, and the tensile yield strength Rp0.2 (strain value for a plastic deformation of 0.2%) of the products in T9 temper are globally significantly higher than those of the products in T8 or T6 tempers. Moreover, the mechanical properties, in particular Rp0.2, increase with the controlled stretching (T6<T8-3%<T8-5%<T9-3%<T9-5%).

A Mn content of the Al—Mg—Li alloy of about 0.4% by weight (alloy B) makes it possible to significantly improve the mechanical resistance (Rp0.2 and Rm), in particular in the direction L, of the alloy in relation to that of an alloy having a Mn content of about 0.14% by weight (alloy A).

Example 2

Several unprocessed forms made of Al—Mg—Li alloy of which the composition is given in the table 1 of the preceding example were cast. Alloys A and B both have a composition suitable for the implementation of the method according to the invention.

Billets 358 mm in diameter were carried out in the unprocessed forms. They were heated to 430-440° C. then hot worked by extrusion on a press in the form of a flat bar (100 mm×10 mm). The products extruded as such were quenched with air (press quenching). They were then subjected to:

-   -   for products in final T6 temper: a two-step artificial aging         carried out for 30 h at 120° C. followed by 10 h at 100° C.;     -   for products in final T8 temper: a controlled stretching with         permanent working set of 3 or 5% (respectively T8-3% and T8-5%)         then a two-step artificial aging carried out for 30 h at 120° C.         followed by 10 h at 100° C.;     -   for products in final T9 temper: a two-step artificial aging         carried out for 30 h at 120° C. followed by 10 h at 100° C. then         a controlled stretching with permanent working set of 3 or 5%         (respectively T9-3% and T9-5%).

Cylindrical samples 4 mm in diameter were tested in order to determine their static mechanical properties (tensile yield strength, R_(p0.2), in MPa; ultimate tensile strength, R_(m), in MPa and elongation, A, in %).

The results obtained are given in tables 4 (direction L) and 5 (direction TL) hereinbelow.

TABLE 4 Mechanical properties of the products obtained (direction L). Mechanical Alloy property T6 T8 (3%) T8 (5%) T9 (3%) T9 (5%) A Rm (MPa) 421 439 441 444 453 Rp0.2 (MPa) 286 319 330 381 398 A (%) 13 11 11 9 10 B Rm (MPa) 439 453 456 451 462 Rp0.2 (MPa) 309 337 352 383 424 A (%) 11 10 8 9 6

TABLE 5 Mechanical properties of the products obtained (direction TL). Mechanical Alloy property T6 T8 (3%) T8 (5%) T9 (3%) T9 (5%) A Rm (MPa) 400 428 417 430 438 Rp0.2 (MPa) 267 300 303 334 345 A (%) 16 17 15 14 14 B Rm (MPa) 423 437 447 448 449 Rp0.2 (MPa) 290 315 323 339 371 A (%) 16 16 16 14 12

The tensile yield strength (strain value for a plastic deformation of 0.2%, Rp0.2) of the products in T9 temper is significantly higher than those of the products in T8 or T6 tempers. Moreover, Rp0.2 increases with the increase in the controlled stretching stress (T6<T8-3%<T8-5%<T9-3%<T9-5%).

A Mn content of the Al—Mg—Li alloy of about 0.4% by weight (alloy B) makes it possible to significantly improve the mechanical resistance of the alloy (Rp0.2 and Rm) in relation to that of an alloy having a Mn content of about 0.14% by weight (alloy A).

The toughness of the products was characterized by the K1c test according to standard ASTM E399. The tests were conducted with a specimen CT8 (B=8 mm, W=16 mm) sampled at mid-thickness. The values of K_(Q) were still invalid according to standard ASTM E399, in particular in relation to criterion P_(max)/P_(Q)≦1.10. For this, the results are presented in K_(max) (stress intensity factor corresponding to the maximum force P_(max)). The results are reported in tables 6 and 7 and illustrated in FIGS. 2 and 3 (specimens L-T and T-L respectively). These results are the averages of at least two 2 values.

TABLE 6 Results of the toughness tests on specimens L-T (K_(max) and K_(Q) in MPa√m) Alloy Toughness T8 (3%) T8 (5%) T9 (3%) T9 (5%) A K_(max) L-T 32.2 33.6 32.5 25.9 K_(Q) L-T 24.6 26.3 25.2 22.3 B K_(max) L-T 30.0 32.2 25.9 — K_(Q) L-T 24.6 26.2 22.0 —

TABLE 7 Results of the toughness tests on specimens T-L (K_(max) and K_(Q) in MPa√m) Alloy Toughness T8 (3%) T8 (5%) T9 (3%) T9 (5%) A K_(max) T-L 30.8 29.5 29.9 — K_(Q) T-L 25.4 25.0 24.7 — B K_(max) T-L 28.5 28.2 25.2 — K_(Q) T-L 24.2 24.3 22.4 —

The products according to the invention have a satisfactory toughness regardless of the Mn content of the alloy.

FIG. 2 shows the tensile yield strength, Rp0.2, of the products of this example according to the toughness, K_(Q) (all of the values of K_(Q) are invalid due to the criterion P_(max)/P_(Q)≦1.10). FIG. 3 shows the tensile yield strength, Rp0.2, of the products of this example according to the stress intensity factor corresponding to the maximum stress, Kmax.

The products in T9 have an excellent compromise between their static properties, in particular Rp0.2, and their toughness, K_(Q), or their stress intensity factor corresponding to the maximum force, Kmax.

The delamination was quantified in a semi-quantitative manner on the fracture surfaces of specimens K1c described hereinabove according to a score from 0 to 2: score 0=absence of visible delamination, score 1=low delamination, score 2=marked delamination (several plates/secondary cracks in the direction L visible). Tables 8 and 9 summarize the scores assigned to the various specimens (specimens L-T and T-L respectively).

TABLE 8 Evaluation of the delamination on specimens L-T (scores) Alloy T8 (3%) T8 (5%) T9 (3%) T9 (5%) A 1 2 2 1 B 0 1 0 —

TABLE 9 Evaluation of the delamination on specimens T-L (scores) Alloy T8 (3%) T8 (5%) T9 (3%) T9 (5%) A 0 1 1 — B 0 0 0 —

The products made of alloy B have a lower delamination than the products made of alloy A. 

1. Method for manufacturing a wrought product wherein: (a) an unprocessed form of aluminum alloy is cast, which has the composition, in % by weight: Mg: 4.0-5.0; Li: 1.0-1.8; Zr: 0.05-0.15; Mn: ≦0.6; Ag: ≦0.5; Fe: ≦0.1; Ti: <0.15; Si: ≦0.05; other elements ≦0.05 each and ≦0.15 in association; the remainder is aluminum; (b) optionally, said unprocessed form is homogenized; (c) said unprocessed form is hot worked to obtain a hot-worked product; (d) optionally, said hot-worked product is solution heat treated at a temperature from 360° C. to 460° C., optionally from 380 to 420° C., for 15 minutes to 8 hours; (e) said hot-worked product is quenched; (f) optionally, said hot-worked and quenched product is straightened or flattened; (g) said hot-worked and quenched product is artificially aged; (h) the aged as such worked product is cold worked under controlled conditions to obtain permanent cold working set in stretching from 1 to 10%, optionally from 2 to 6%, optionally from 3 to 5%.
 2. Method according to claim 1 wherein the hot working of (c) is a working by extrusion of the unprocessed form.
 3. Method according to claim 1 wherein the hot working of (c) is carried out at an initial temperature greater than 400° C., optionally from 420° C. to 450° C.
 4. Method according to claim 1 wherein the quenching of the (e) is a press quenching.
 5. Method according to claim 1 wherein the quenching of the (e) is carried out with air.
 6. Method according to claim 1 wherein the artificial aging of the product hot worked and quenched in (g) is carried out by heating, in one or several steps, at a temperature less than 150° C., optionally at a temperature from 70° C. to 140° C., for 5 to 100 hours.
 7. Method according to claim 1 wherein said unprocessed form made of aluminum alloy has a Mn content, in % by weight, from 0.2 to 0.6, optionally from 0.35 to 0.5.
 8. Method according to claim 1 wherein said unprocessed form made of aluminum alloy has a Zn content, in % by weight, less than 0.04%, optionally less than or equal to 0.03%.
 9. Method according to claim 1 wherein said unprocessed form made of aluminum alloy has an Fe content, in % by weight, less than 0.08%, optionally less than or equal to 0.07%, optionally less than or equal to 0.06%.
 10. Method according to claim 1 wherein said unprocessed form made of aluminum alloy has a Li content, in % by weight, less than 1.6%, optionally less than or equal to 1.5%, optionally less than or equal to 1.4%.
 11. Wrought product able to be obtained according to a method of claim
 1. 12. Wrought product according to claim 11 having at mid-thickness, for a thickness between 0.5 and 15 mm, at least one static mechanical resistance property among the properties (i) to (iii) and at least one damage tolerance property among the properties (iv) to (v): (i) an ultimate tensile strength, Rm (L)≧440 MPa, optionally Rm (L)≧445 MPa and, optionally, Rm (L)≧450 MPa; (ii) a tensile yield strength Rp0.2 (L)≧360 MPa and optionally Rp0.2 (L)≧380 MPa and, optionally, Rp0.2 (L)≧400 MPa; (iii) a tensile yield strength Rp0.2 (TL)≧330 MPa and optionally Rp0.2 (TL)≧340 MPa and, optionally, Rp0.2 (TL)≧370 MPa; (iv) a toughness, measured according to standard ASTM E399 with specimens CT8 of width W=16 mm and of thickness=8 mm, KQ (L-T) 20 MPa√m, optionally KQ (L-T)≧22 MPa√m; (v) a stress intensity factor corresponding to the maximum force Pmax, measured according to standard ASTM E399 with specimens CT8 of width W=16 mm and of thickness=8 mm, Kmax (L-T)≧20 MPa√m, optionally Kmax (L-T)≧25 MPa√m.
 13. A wrought product obtained according to a method of claim 1 that is used in an aircraft structural element, optionally a fuselage skin, a circumferential frame, a fuselage stiffener or stringer or a wing skin, a wing stiffener, a rib or a spar. 